1. Field of the Invention
The present invention relates to a fixing apparatus for use in electrostatic copying or printing machines.
2. Description of the Prior Art
Hitherto, the heat fixing method has been used most widely in electrophotographic copying or printing machines. According to the heat fixing method, a toner image electrostatically adhered to the surface of a supporting member such as a sheet of paper is fused with heat to permanently fix the toner image on the supporting member.
However, it has been found that such a heat fixing method has various drawbacks. Firstly, it needs a large quantity of electric power for the heat source. The quantity of electric power consumed in this heat fixing station reaches about 70-80% of the total power consumption by the whole copying machine. Secondly, it takes a relatively long time to warm up the fixing apparatus. Therefore, it is impossible to start a copying operation immediately after the power source of the copying machine is switched on. Lastly, heat emitted from the fixing apparatus has an adverse effect on the characteristics of the photosensitive medium and/or developer. The whole copying machine suffers from the heat emitted from the fixing apparatus and there occurs a change in the characteristics of the photosensitive medium and/or developer which in turn reduces the efficiency and effective life thereof. In the worst case, there may occur a serious problem of the supporting member catching fire when it becomes jammed in the fixing apparatus.
Due to the above mentioned drawbacks involved in heat fixing method, nowdays there is an increased tendency to use the pressure fixing method instead of the heat fixing method. According to the pressure fixing method, toner particularly prepared for the pressure fixing method is adhered onto a supporting member to form a toner image and the supporting member carrying thereon the toner image is passed between a pair of rollers contacting with each other under high pressure so as to fix the toner image with pressure.
A typical example of pressure fixing apparatus according to the prior art is shown in FIG. 1. Designated by P is a supporting member carrying thereon a toner image. Fixing of the toner image is effected between a pair of rollers 1 and 2 under pressure. The necessary pressure is given by a pressing mechanism 3 which brings the two rollers 1 and 2 into contact with each other under a high pressure. One of the rollers which contacts with the image side surface of the supporting member P, that is, the roller 1 is made of metal. The other roll which contacts the backside surface of the supporting member P, that is, the roller 2 is made as an elastic roller by covering the surface of the roller with elastic material. As an alternative, both of the rollers 1 and 2 may be made as rigid metal rollers. In this case, the two metal rollers 1 and 2 are disposed in such manner that the one roller intersects the other.
The pressure fixing apparatus comprising a metal roller and an elastic roller has such disadvantage that the maximum pressure which the fixing rollers can apply to the toner image supporting member is limited by the kind of material used for the elastic roller. Therefore, it is impossible to apply a high pressure to this type of the fixing rollers. Moreover, the elastic roller is easily damaged by toner and carrier.
For the latter mentioned type of pressure fixing apparatus comprising two rigid metal rollers, it is possible to apply a high contact pressure. Nevertheless, it has some drawbacks in practical use. The fixing roller is required to have a larger diameter in order to apply onto the surface of the supporting member a load (pressure) more than the critical pressure necessary for sufficiently fixing the toner image. When this requirement is satisfied, the apparatus inevitably becomes large in size and heavy in weight as a whole. This will also have an unfavourable effect on the fixability of the toner image. Moreover, if the two metal rollers which must be arranged intersect each other as previously noted have different diameters, then there will arise a problem of creasing in the supporting member. This problem of creasing becomes remarkable particularly when a supporting member of larger size in width is used.
We, the inventors of the present invention have carried out a number of experiments on the pressure fixing apparatus. These inventive experiments have led us to the finding that an important factor affecting the pressure fixability of toner image is the maximum pressure per unit area (Po) rather than the total pressure exerted on the nip portion between the pair of rollers (Wo). In other words, it has been found that the local maximum pressure Po has a greater effect on the pressure fixability of the toner image than the total pressure Wo does.
As seen in FIG. 2, the conventional pressure fixing apparatus can not produce a high local maximum pressure Po even when the total pressure Wo is increased. The rollers 1 and 2 shown in FIG. 2 have a larger diameter Ro. A toner image supporting member P is nipped in between the two rollers and there is formed a nip width (angle .alpha.). As will be easily understood, the nip width will become broader with the increase of the roller diameter Ro. Therefore, the increase of the total pressure Wo can not bring about the corresponding increase of the value of Po (maximum pressure per unit area). The increase of the value Po is relatively small or the value of Po remains unchanged as compared with the increase of the total pressure Wo. For this reason, in the case of the conventional pressure fixing apparatus, no substantial improvement in pressure fixability can be attained by increasing the total pressure Wo.